Phenolphthalein is widely used as a cathartic and is the basis for many laxatives. Phenolphthalein was nominated for further toxicity and carcinogenicity studies in rodents because adequate studies had not previously been performed, and significant numbers of people are exposed to this compound through its use as a laxative. The objective of this contract is to administer the compound in the feed to F344/N rats and B6C3F1 mice for two years, and to examine tissues and organs for the presence of treatment-related non-neoplastic or neoplastic lesions.